The present invention relates to display devices, such as devices driving an active matrix electrophoretic display by varying the common voltage.
Displays, such as liquid crystal (LC) and electrophoretic displays include particles suspended in a medium sandwiched between a drive or pixel terminal and a common terminal. The pixel terminal can be controlled in various ways. The most simple and low cost way is direct control of the pixel electrode by a display controller. This is called a direct-drive or segmented display, where every pixel (also called a segment in this type of display) is under the direct control of the controller. Another way to control the pixel terminal is by way of passive-matrix driving, where the pixel terminals are connected to each other in rows and the common terminals are connected to each other in columns, where each row and column are under the direct control of a display controller. This way a simple matrix display is formed. This is commonly used for simple LC matrix displays. For electrophoretic media this way of driving does not work, as a requirement for passive matrix driving is a voltage threshold around 0V where the medium does not switch. This is absent in electrophoretic media. The common way to drive larger, higher resolution displays is by means of active-matrix addressing. In that case the pixel terminal includes pixel drivers, such as an array of thin film transistors (TFTs) that are controlled to switch on and off to form an image on the display. This conventional method of driving a display is referred to as scan line driving. The voltage difference between a TFT or the pixel terminal and the common terminal, which is on the viewer's side of the display, causes migration of the suspended particles, thus forming the image. Displays with an array of individually controlled TFTs or pixels are referred to as active-matrix displays.
In order to change image content on an electrophoretic display, such as from E Ink Corporation for example, new image information is written for a certain amount of time, such as 500 ms to 1000 ms. As the refresh rate of the active-matrix is usually higher, this results in addressing the same image content during a number of frames, such as at a frame rate of 50 Hz, 25 to 50 frames. Electrophoretic active matrix displays are applied in many applications such as e-readers. Although this text refers generally to E Ink as examples of electrophoretic displays, it is understood that the invention can be applied to electrophoretic displays in general, such as e.g. SiPix, where the microcups are filled with white particles in a black fluid.
Circuitry to drive displays, such as electrophoretic displays, are well known, such as described in U.S. Pat. No. 5,617,111 to Saitoh, International Publication No. WO 2005/034075 to Johnson, International Publication No. WO 2005/055187 to Shikina, U.S. Pat. No. 6,906,851 to Yuasa, and U.S. Patent Application Publication No. 2005/0179852 to Kawai; U.S. Patent Application Publication No. 2005/0231461 to Raap; U.S. Pat. No. 4,814,760 to Johnston; International Publication No. WO 01/02899 to Albert; Japanese Patent Application Publication Number 2004-094168 and WO2008/054209 and WO2008/054210 to Markvoort, each of which is incorporated herein by reference in its entirety.
Conventional active matrix E-ink displays suffer from various drawbacks. One drawback is that power consumption during an image update is relatively large, due to the relatively high voltages that must be applied during addressing of the display. A straightforward solution would be lowering the addressing voltages. However, the disadvantage of the lower voltage levels is that the image update time increases more than linear with the voltage reduction, leading to very long image update times (i.e., slower image updates). Another drawback is that the image update time of E-ink is relatively long despite the high voltage levels.
WO 2008/054209 referred to herein above discloses a display without an increased image update time, wherein the voltages on the active matrix and thereby the power consumption are decreased. The drive scheme uses a voltage not equal to zero on the common electrode of the display to reduce the voltages needed in the rows and columns of the active matrix. During a first period, wherein the pixels may be brought in an extreme pixel state corresponding to a first color (e.g. white or black), the voltage on the common electrode has a first polarity. The pixels now can be brought in the first color state by providing a column voltage with an opposite second polarity. The pixels that should not be brought in the first color state are provided with a column voltage having the same polarity and value as the voltage on the common electrode. As a result, there is a “stable transition”, wherein there is no voltage over the pixel and the color of the pixels concerned does not change. Thereby, image artifacts can be avoided. During a second period, wherein the pixels may be brought in an extreme pixel state corresponding to a second color (black if the first color is white and white if the first color is black), the voltage on the common electrode has the second opposite polarity. The pixels now can be brought in the second color state by providing a column voltage with the opposite first polarity. The pixels that should not be brought in the second color state are provided with a column voltage having the same polarity and value as the voltage on the common electrode, resulting in a stable transition without color change for those pixels.
Although the power consumption of the display known from WO2008/054209 is reduced with respect to other prior art display devices, it is desirable to reduce it even further, in particular to conserve battery life of mobile products.